A stem type femoral hip replacement prosthesis comprises an elongated stem for placement in the intramedullary cavity of the femur and a head defining the proximal end of the prosthesis for replacing proximal portions of the natural femur. The head of the prostheses is substantially spherical and is pivotally engageable in an acetabular cup which defines a separate portion of a prosthetic system which is affixed to the natural acetabular. The stem type femoral prosthesis may further include a collar for engagement against the resected proximal end of the natural femur, and a neck extending from the collar to the head. Most prior art stem type femoral prostheses have been of unitary construction. However many prior art stem type prostheses have been of modular construction with any of a plurality of heads being selectively engageable on any of a plurality of necks. The particular head and neck combination has been selected to achieve optimum fit in the patient. Examples of prior art femoral stem type prostheses are shown in U.S. Pat. No. 4,752,296 which issued to the inventors herein on Jun. 21, 1988.
The best length and diameter of the stem of a stem type femoral hip replacement prostheses is dependent on the patient's physiology and pathology. For example, a small patient with considerable disuse atrophy of the femur may have a wide intramedullary cavity or femoral canal and thus may require a prostheses with a small proximal end but a large stem diameter for proper fit in the intramedullary cavity of the femur. In other instances, an extra long stem may be needed to span a femoral fracture or other defect, and thus allow the prosthesis to act as a support for the defect while healing occurs
Typically the prosthesis, and particularly the stem of the prosthesis, have been custom made to accommodate the specific physiological and pathological needs of the patient. This typical prior art approach has at least three major disadvantages. In particular: the custom made prosthesis is very expensive; there is considerable time required to obtain the prosthesis, during which time the patient can be adversely affected; and, fitting the prosthesis from x-ray data is not completely reliable, and the custom made prosthesis may in fact not fit well, if at all.
In view of the preceding problems, some prior art prostheses have been developed with modular stems to alter the length of the stem in accordance with the particular needs of the patient. A stem extension of a selected length can be added to the proximal portion of the stem type prosthesis to allow fitting intraoperatively. These prior art modular stem prostheses generally have employed a conical taper fit between the proximal portion of the prosthesis and the extension. Some such prior art modular stem type prostheses have employed a screw to force the mating tapered ends tightly together.
The interengagement of the mating male and female tapered components of the prior art modular stem type prosthesis produces substantial tension forces on the surface of the female component. More particularly, the forceful urging of the component having the male taper into the component having the female taper urges the female tapered portion outwardly to generate the substantial tension forces on the surface of that member. These substantial tensile forces necessarily occur at critical surface areas near the stem-extension interface. The tension forces developed in this context can be compared to the hoop stresses created on the hoops of a barrel. Unlike a barrel, however, the prothesis is repeatedly subjected to bending stresses during normal usage. During instances of such bending stress, the portion of the prothesis having the female taper will be subjected to complex tension forces caused both by the wedging action of the mating tapers (e.g, hoop stress) and by the bending stresses. Such surface tensile stresses are highly undesirable in that they contribute substantially to fatigue of the prosthesis, and thus substantially weaken the stem/extension composite structure at the structurally critical interface of the stem and the stem extension. In particular, microfissures or microcracks in the surface of the component having the concave taper can experience accelerated propagation when subjected to additional tensile forces in response to the bending stresses exerted on these critical regions of the prostheses during normal usage.
In view of the above, it is an object of the subject invention to provide a modular prosthesis that enable optimum fitting to the patient.
It is another object of the subject invention to provide a modular prosthesis that reduces tensile forces significantly in critical areas of the prosthesis.
It is a further object of the subject invention to provide a modular prosthesis having a stem and a stem extension which minimize tensile forces at surface regions adjacent the interface of the stem and the extension of the prostheses.